1. Field of the Invention
This invention relates to contacting a hydrocarbon gas stream with a preferential physical solvent. It particularly relates to separating and recovering ethane and higher boiling hydrocarbons from a hydrocarbon gas stream. It more specifically relates to supplying specific market needs for hydrocarbons by selective recovery of C.sub.2 +, C.sub.3 +, C.sub.4 +, or C.sub.5 + hydrocarbons from a hydrocarbon gas stream.
2. Review of the Prior Art
Hydrocarbons must often be recovered from such hydrocarbon gas streams as natural gas, alkylates, reformates, catalytic hydroconversion effluents, hydrofining effluents, and the like. Many recovery processes are available, but countercurrently contacting the upwardly flowing gas stream with a downwardly flowing liquid under conditions furnishing high interfacial surface area is often a preferred recovery process, known as absorption when the liquid is a solvent generally or extraction when the liquid is a preferential physical solvent.
Most physical solvents show some preference among hydrocarbons in a mixture thereof. In other words, they have greater solvency, perhaps because of a stronger physical attraction, for one or more hydrocarbons in such a mixture. This preference is measured by the absorption principle, leading to an alpha or relative volatility. Most of the commonly used lean oils, for example, have relative volatilities of methane over ethane of slightly less than 5.
Lean oils have been used in absorption plants for extracting C.sub.4 + hydrocarbons, with some recovery of propane, from hydrocarbon gas streams for many years. The lean oils are non-selective for lighter hydrocarbons, such as ethane and propane, so that relatively large amounts of methane are absorbed, thereby making the separation of ethane and propane from methane quite difficult and expensive. Due to the market demand for lighter hydrocarbons, such as ethane and propane, and the lack of selectivity of lean oils for such components, the absorption processes have been replaced by processes consisting of refrigerated oil absorption, simple refrigeration, cascaded refrigeration, Joule-Thompson, or cryogenic expander processes. The related Mehra Process patents and applications are directed toward use of selected physical solvents having strongly preferential characteristics in absorption/flashing and extractive-stripping steps. The Mehra Process thereby overcomes the disadvantages of non-selectivity of common lean oils for lighter hydrocarbons, such as ethane and propane.
Another disadvantage of the older processes for recovering hydrocarbons from hydrocarbon gas streams is that the recovery levels are quite inflexible. In contrast, the Mehra Process overcomes the inflexibility drawback by effectively utilizing the selectivity and volatility characteristics of preferential physical solvents. Typical recoveries for these processes are compared in Table I, including the extractive-stripping embodiment of the Mehra Process.
TABLE I ______________________________________ COMPARISON OF TYPICAL LIQUID RECOVERIES ETH- PRO- BU- GASO- ANE PANE TANES LINE EXTRACTION (%) (%) (%) (%) ______________________________________ ABSORPTION 5 25 75 87 REFRIGERATED 15 75 90 95 ABSORPTION SIMPLE 25 55 93 97 REFRIGERATION CASCADED 70 85 95 100 REFRIGERATION JOULE-THOMPSON 70 90 97 100 EXPANSION TURBO-EXPANDER 85 97 100 100 MEHRA PROCESS 2-90 2-99 2-100 100 ______________________________________
In summary, the oil absorption, refrigerated oil absorption, simple refrigeration, and cascaded refrigeration processes operate at the pipeline pressures, without letting down the gas pressure, but the recovery of desirable liquids (ethane plus heavier components) is poor, with the exception of the cascaded refrigeration process which has extremely high operating costs but achieves good ethane and propane recoveries. The Joule-Thompson and cryogenic expander processes achieve high ethane recoveries by letting down the pressure of the entire inlet gas, which is primarily methane (typically 80-85%), but recompression of most of the inlet gas is quite expensive.
In all of the above processes, the ethane plus heavier components are recovered in a specific configuration determined by their composition in the raw hydrocarbon gas stream and equilibrium at the key operating conditions of pressure and temperature within the process. Under poor economic conditions when the ethane price as petrochemical feedstock is less than its equivalent fuel price and when the propane price for feedstock usage is attractive, for example, the operator of a hydrocarbon gas liquid extraction plant is limited as to operating choices because he is unable to minimize ethane recovery and maximize propane recovery in response to market conditions.
The extractive-flashing embodiments of the Mehra Process, as disclosed in U.S. Pat. Nos. 4,421,535, 4,511,381 and 4,526,594, utilize preferential physical solvents for processing natural gas streams by extracting, flashing, compressing, cooling, and condensing the desired components for producing natural gas liquid products. The extractive-flashing embodiments of the Mehra Process combine the advantages of the higher-pressure extraction processes by selectively recovering and letting down the pressure of essentially the desired components, thereby reducing the compression of undesirable components, such as methane, while achieving high levels of component recovery in a flexible manner.
Under the heading, "New NGL Extraction Process", this embodiment of the Mehra Process is described on pages 7 and 8 of Gas Processors Report, P.O. Box 33002, Tulsa, Ok. 74153. Commonly owned Ser. No. 759,327 is particularly directed toward processing of nitrogen-rich natural gas streams in this manner.
The extractive-stripping embodiment of the Mehra Process is taught in commonly owned Ser. No. 784,566 and Ser. No. 808,463, which are wholly incorporated herein by reference. This process embodiment utilizes an extractive-stripping (ES) step and eliminates the need for flashing of the rich solvent stream to separate the desired components of a raw gas stream. Residue gas, consisting primarily of methane, leaves the top of the Extractor/Stripper (ES) column while the rich solvent containing the desired hydrocarbon components leaves the bottom of the ES column. The C.sub.2 + hydrocarbons are then separated as the natural gas liquid product from the top of the hydrocarbon product column, and the separated solvent is recycled to the ES column for reuse. This process can selectively recover C.sub.2 +, C.sub.3 +, C.sub.4 +, or C.sub.5 + hydrocarbons at high recovery levels while rejecting lower-molecular weight hydrocarbons into the residue gas stream.
The rich solvent leaving the bottom of the ES column is let down in pressure to a pressure level consistent with the operation of the product column. This pressure level also obviates the need for a downstream compressor or pump. The rich solvent may be economically heated before entering the product column in order to lower the reboiler heat load and improve separation of hydrocarbons from the physical solvent.
The product column is a typical fractionation-type column in which the selectively extracted hydrocarbons are separated from the preferential physical solvent. The desired hydrocarbons are recovered from the top of the product column as an overhead stream while the hot, lean solvent leaves the bottom of the product column. The temperature at the bottom of the product column is selected to ensure the recovery of all desirable hydrocarbons and is no higher than the boiling point of the physical solvent at the operating pressure. In order to minimize loss of the physical solvent with the product, the column overhead is refluxed with a portion of the condensed hydrocarbons.
In order to minimize energy consumption, the hot, lean physical solvent, leaving the bottom of the product column, is effectively utilized for heating the rich solvent feed to the product column and for reboiling the ES column before returning to the top of the extraction section of the ES column as cool, lean preferential physical solvent.
The rich solvent, leaving the bottom of the ES column, contains only the specified amounts of the undesirable lighter components, such as C.sub.1 in C.sub.2 + products, in order to meet the product specifications (NGL specifications if the hydrocarbon gas is natural gas). Because such a purity requirement has been combined with selectivity in the ES column, wherein the selection capability of operating pressure is unavailable because it is generally determined by the delivery pressure of the residue gas, only temperature flexibility at the bottom of the ES column is available for meeting the required specification of undesirable components because the other flexibility of flow rate of preferential physical solvent to the ES column is effectively utilized in meeting the selective recovery levels of desired hydrocarbon components of the raw gas stream. The reboiler and the stripping section in the bottom portion of the ES column provide additional selectivity, thereby gaining one more degree of freedom which is effectively utilized by appropriately selecting the reboiling temperature in order to produce the desired rich solvent stream.
Suitable preferential physical solvents, as disclosed in Ser. No. 784,566 and Ser. No. 808,463, are rich in C.sub.8 -C.sub.10 aromatic compounds having methyl, ethyl, or propyl aliphatic groups, including mesitylene, n-propyl benzene, n-butyl benzene, o-xylene, m-xylene, p-xylene, and mixtures thereof, rich being defined as more than 15% by weight. Substantially pure mesitylene is preferred. Suitable sources of these C.sub.8 -C.sub.10 aromatic compounds are aromatic streams, such as in petroleum refineries that are rich in mixed xylenes, C.sub.9 alkylaromatics, and other C.sub.8 -C.sub.10 aromatics. These compounds boil in the range of 270.degree.-425.degree. F. and are stable at the process temperatures used in separating mixtures into useful fractions and/or components, such as in distillation, extractive stripping, and extractive distillation operations. Moreover, they are also hydrocarbons which can be left in the liquid products in trace amounts, without interfering with use of such products in gasoline, for example, so that purification of the liquid products is not needed.
A principal refinery source of C.sub.8 -C.sub.10 aromatic feed streams may be found in catalytically reformed naphthas in which a C.sub.9 heart cut or extract of the reformate is enriched in C.sub.9 alkylbenzenes, a typical reformate containing as much as 57% trimethylbenzenes based on the total content of C.sub.9 aromatics. The composition of a C.sub.9 heart cut is typically about 2.5, 87.5 and 10 mole % of C.sub.8, C.sub.9 and C.sub.10 aromatics, respectively. Other sources of C.sub.8 -C.sub.10 aromatic feedstocks are derived from gasoline producing processes such as the conversion of methanol to gasoline, as described in U.S. Pat. Nos. 3,931,349, 3,969,426, 3,899,544, 3,894,104, 3,904,916 and 3,894,102, and the conversion of synthesis gas to gasoline as described in U.S. Pat. Nos. 4,096,163, 4,279,830, 4,304,871 and 3,254,023, all of which are incorporated by reference. A C.sub.7 -C.sub.9 mixed aromatic feedstock also may be used and can be derived from various sources including petroleum refinery sources, pyrolysis of coal to produce coke, tar sands, etc.
In petroleum processing operations such as transalkylation, isomerization, and disproportionation, for example, the product streams so produced are further treated, by fractionation and the like, to obtain alkylaromatic streams which contain substantial quantities of alkylbenzenes such as toluene, xylenes, and trimethylbenzenes. A typical alkylaromatic fraction which may be obtained contains predominantly C.sub.7 to C.sub.9 hydrocarbons and is referred to as crude xylenes.
Refinery streams suitable as preferential physical solvents for the present process are C.sub.9 alkylaromatics, a C.sub.7 to C.sub.9 mixture of alkylaromatics, or a C.sub.8 -C.sub.10 mixture of alkylaromatics. The C.sub.9 alkylaromatic hydrocarbons are characterized as mainly monocyclic aromatic compounds, such as alkylbenzenes, which have at least one alkyl group which preferably contains no more than 4 carbon atoms. The C.sub.9 aromatic hydrocarbons include, for example, 1,2,3-trimethylbenzene (hemimellitene), 1,2,4-trimethylbenzene (pseudocumene), 1,3,5-trimethylbenzene (mesitylene), isopropylbenzene (cumene), 1,2-methylethylbenzene, 1,3-methylethylbenzene, and 1,4-methylethylbenzene.
The C.sub.9 alkylaromatics for use in the present process are conveniently available as product streams from various petroleum processing operations, including gasoline producing processes such as the conversion of methanol to gasoline or the conversion of carbon monoxide and hydrogen (syngas) to gasoline. Catalytic reformates, for example, are particularly preferred since they are enriched in aromatics and the C.sub.9 fraction can be readily separated from non-aromatics by extraction with aqueous glycols, typically a Udex unit. The typical composition of extracted C.sub.9 reformate and the boiling points of the C.sub.9 aromatics contained therein are shown below in Table II.
TABLE II ______________________________________ COMPOSITION OF C.sub.9 AROMATICS IN EXTRACTED REFORMATE Wt. % Boiling Freezing (based on Point Point total C.sub.9 Compound (.degree.F.) (.degree.F.) aromatics) ______________________________________ API Gravity -- IBP, .degree.F. -- EBP, .degree.F. -- Isopropylbenzene 306 -141 0.6 n-Propylbenzene 319 -147 5.2 m-Ethyltoluene 322 -140 17.4 p-Ethyltoluene 324 -80 8.6 1,3,5-Trimethylbenzene 329 -49 7.6 (mesitylene) o-Ethyltoluene 329 -114 9.1 1,2,4-Trimethylbenzene 337 -47 41.3 (pseudocumene) 1,2,3-Trimethylbenzene 349 -14 8.2 (hemimellitene) Indane 352 -- 2.0 100.0% ______________________________________
While the quality of crudes may affect the quantity and type of C.sub.9 aromatics extracted from a naphtha reformate, about 57 wt. % of the total C.sub.9 aromatics are trimethylbenzenes in which pseudocumene, mesitylene and heminellitene are typically produced in the following ratios:
Pseudocumene=1 PA1 Mesitylene=0.18 PA1 Hemimellitene=0.20
The C.sub.9 aromatics may be further characterized as having an initial boiling point range of 230.degree.-280.degree. F., an end boiling point range of 350.degree.-425.degree. F., and an API gravity of 35-60.
A useful, although not ideal, source of preferential physical solvent is primarily a mixture of seven to nine carbon atom alkyl aromatics which include C.sub.7 and C.sub.8 aromatics, such as toluene, ethylbenzene and xylenes, and C.sub.9 alkylaromatics identified in Table II above. Such charge stocks may also be derived from catalytic reformates, pyrolysis gasoline, etc., by distillation and solvent extraction to separate aromatic compounds from aliphatics. Other sources of suitable charge stocks include crude xylene streams, which actually contain alkylaromatics having 7 to 9 carbon atoms, and effluents from toluene transalkylation reaction zones which contain benzene, xylene, C.sub.9 aromatics, and aromatics heavier than C.sub.9. Mixtures of toluene and C.sub.9 alkylaromatics may also be employed. The composition of a typical C.sub.7 -C.sub.9 reformate cut is shown below in Table III.
TABLE III ______________________________________ Products: Analysis wt. percent ______________________________________ Naphthenes 0.15 Benzene 2.03 Toluene 19.69 Ethylbenzene 0.004 Paraxylene 12.04 Metaxylene 27.64 Orthoxylene 10.40 p-Ethyltoluene 0.02 m-Ethyltoluene 0.06 o-Ethyltoluene 0.01 Mesitylene 7.18 Pseudocumene 15.82 Hemimellitene 1.93 Ethylxylenes 0.13 Durene 1.19 Isodurene 1.43 Prehnitene 0.28 ______________________________________
The C.sub.7 to C.sub.9 aromatic mixture may be further characterized as having an initial boiling point range of 150.degree. F., an end boiling point range of 350.degree. F., and an API gravity of about 40.
If the hydrocarbon gas stream is sour, it is preferred, in the processes of Ser. No. 784,566 and Ser. No. 808,463, that it be sweetened by contact with an acid-absorbing solvent, such as an amine, for example, before the extraction-stripping process of this invention is utilized. However, if an amine pretreating step is not suitable, the sour hydrocarbon gas stream can be treated according to the instant process. The acidic components are then maintained in liquid-phase or vapor-phase solution or contact, respectively, with the heavier hydrocarbon components until the solution or mixture, as a liquid, can be contacted by an acid-absorbing solvent. Because such post-absorption sweetening is done in liquid phase, the capital cost for equipment is relatively low.
However, when processing must be done without an intermediate flashing stage between the extractor-stripper column and the product column or when extraction must be done at pressures of 400 psia or more or when the inlet gas is quite lean in C.sub.2 + hydrocarbons, it becomes difficult to make a clean split between methane as the residue gas and C.sub.2 + hydrocarbons as product or between methane and ethane as the residue gas and C.sub.3 + hydrocarbons as product or between C.sub.1 +C.sub.2 +C.sub.3 hydrocarbons as the residue gas and C.sub.4 + hydrocarbons as product. Such situations become doubly acute when the pressure is high at the same time that the inlet gas stream is quite lean. Increasing the temperature, such as to above 600.degree. F., to achieve such selectivity has its limitations because such high temperatures may cause undesired breakdown of the solvent and because operating temperatures above 600.degree. F. make it necessary to derate the strength of carbon steel and use stainless steels or steels having special design criteria, thereby increasing capital costs. It is true that clean splits can be made under such conditions, as disclosed in Ser. No. 784,566 and in Ser. No. 808,463, but they may require high energy penalties and difficult processing procedures such as a very high solvent recirculation rate and/or a very high absorption temperature, such as 600.degree.-700.degree. F.
A method for efficiently separating consecutively numbered hydrocarbons in a gas stream without recourse to energy-intensive steps and/or difficult processing procedures, while extractively stripping or distilling a hydrocarbon gas stream with a preferential physical solvent without an intermediate flashing step, is accordingly needed. The following references teach related methods without filling this need.
U.S. Pat. No. 2,570,066 is directed to a method for segregating pure hydrocarbons from hydrocarbon mixtures by distractive distillation in the presence of an aromatic hydrocarbon solvent which is preferably a mono-cyclic aromatic hydrocarbon fraction boiling in the range between 365.degree. and 750.degree. F. Mono-cyclic aromatic hydrocarbons having 10 carbon atoms, exemplified by tetramethylbenzenes such as 1,2,4,5-tetramethylbenzene, 1,2,3,5-tetramethylbenzene, and 1,2,3,4-tetramethylbenzene, and further exemplified by 1,2,-dimethyl-3-ethylbenzene, 1,2-dimethyl-4-ethylbenzene, and the like, are preferred. Durene, isodurene, prehnitene, and mixtures thereof are especially beneficial. The ratios of solvent to feedstock may range from about 1:1 to about 20:1, about 5:1 being preferred.
U.S. Pat. No. 2,573,341 relates to a process for recovering olefinic hydrocarbons and particularly high purity ethylene from coke oven gas, refinery off-gas, and pyrolysis gas, having respective ethylene contents of 4.0, 5.0, and 27.0 mol. %, which are the feedstocks to an absorber-stripper column having a reboiler at its bottom and two intercoolers to remove the heat of extraction. Its overhead is fuel gas, and its bottoms are fed to a succession of distillation columns for separating ethane, ethylene, and propane streams.
U.S. Pat. No. 2,804,448 describes a process in which oil refinery gases from a cracking operation are absorbed with ethane as solvent. The rich solvent is stripped with heat to produce a distillate product. The residue gas is dried and extractively distilled to produce a partially stripped, rich solvent which is stripped with heat to produce a methane-rich gas which is recycled to the inlet gas line ahead of the compressor.
U.S Pat. No. 3,770,622 teaches the use of a physical solvent, having combined oxygen and in which CO.sub.2 and H.sub.2 S are relatively more soluble than methane, for treating wet natural gas mixtures containing CO.sub.2, H.sub.2 S, and hydrocarbons heavier than methane. Such physical solvents include propylene carbonate, N-methyl pyrrolidone, glycerol triacetate, and polyethylene glycol dimethylether. The physical solvents are designated as selective because CO.sub.2 and H.sub.2 S are relatively more soluble therein than methane by a factor of 5 or higher under operating conditions and because they have utility for absorbing liquid hydrocarbons during CO.sub.2 and H.sub.2 S removal from wet natural gas mixtures to enable recovery of gasoline values therefrom by countercurrent absorption at a superatmospheric pressure and a base temperature between -20.degree. F. and 100.degree. F. within the absorption zone. The acid gas (CO.sub.2, H.sub.2 S) and hydrocarbons lighter than propane are then flashed and/or stripped from the liquid stream within the lower pressure zone to separate CO.sub.2, H.sub.2 S, and light hydrocarbons from the organic liquid which is next passed to a settling zone where the organic liquid and the liquid hydrocarbons are separated by gravity.